We have studied the elasticity and pressure-density equation of state of MgO in diamond cells to 55 GPa and have doubled the previous pressure limit of accurate elasticity determinations for crystals. Integrating single-crystal velocity data from Brillouin scattering measurements and density data from polycrystalline x-ray diffraction, we obtained the three principal elastic tensor elements (C11, C12, and C44) and various secondary elasticity parameters, including single-crystal elastic anisotropy, Cauchy relation, aggregate sound velocities, and Poisson's ratio, as functions of pressure. The present study also provides a direct determination of pressure without recourse to any prior pressure standard, thus creating a primary pressure scale. The commonly used ruby fluorescence pressure scale has thus been improved to 1% accuracy by the new MgO scale. E lasticity, the fourth-rank tensor defining the strain of crystalline solids under stress (1), contains key parameters relating the microscopic bonding properties to the macroscopic mechanical behaviors of the solid under compression. The study of elasticity under high pressure has wide applications in fields ranging from crystal physics to seismology (2). Direct experimental measurements of the tensor elements, however, have previously been limited to 32 GPa (3)-above this pressure our knowledge has been derived from theoretical predictions, extrapolations of lower pressure data, or indirect measurements with assumed stress͞strain conditions (4). In the present study, we extend measurements of the elasticity of MgO, an archetypal oxide with the simple NaCl-type (B1) crystal structure, to 55 GPa at 300 K.MgO has been the subject of extensive theoretical and experimental investigations (5-17) that have revealed the fundamental bonding properties in this class of materials. With the wealth of available data, MgO is often used as a test ground for new theories, new experiments, and even as a pressure calibration standard for high-pressure, high-temperature experimentation (18,19). Moreover, MgO is a major component in the Earth's lower mantle (20,21), and knowledge of its high-pressure behavior is crucial for understanding deep Earth geophysics. The crystal structure and volume compression of polycrystalline MgO samples without pressure media have been studied with x-ray diffraction in diamond cells up to 227 GPa (7,22, 23). MgO in the B1 structure is stable over the entire pressure range. Nonhydrostatic conditions in these measurements, however, may introduce systematic deviations in pressure of up to 10% in the pressure-density equations of state (22-24). Ultrasonic and Brillouin scattering studies of single-crystal MgO (8,16,25) have yielded detailed information on its elastic tensor, anisotropy, and aggregate shear and bulk moduli, but have been limited to a maximum pressure of 18.6 GPa. Evidence for nonhydrostaticity has also been found in previous Brillouin scattering experiments at pressures exceeding the hydrostatic limits of the pressure media used in these s...
